1. SNU Mobile Networks Lab. S-MAC (Sensor-MAC) T-MAC (Timeout-MAC) 2004. 4. 6 Kae Won, Choi Kyoung hoon, Kim

2. SNU Mobile Networks Lab. Contents Introduction Periodic Listen and Sleep Collision Avoidance Overhearing Avoidance Message Passing T-MAC

3. SNU Mobile Networks Lab. Introduction Energy-efficiency is primary concern. Sacrifice.. –Node-level fairness Assuming just one application is on the operation. –Latency In-network processing –Long message

4. SNU Mobile Networks Lab. Periodic Listen and Sleep Choosing schedules –The node randomly choose time to go to sleep. –The node receives and follows its neighbor’s schedule by setting its schedule to be the same. –If the nodes receives a different schedule after it selects its own schedule, it adopts its own schedule.

5. SNU Mobile Networks Lab. Periodic Listen and Sleep Maintaining synchronization –Update schedule by sending a SYNC packet periodically. SYNC packet contains address of the sender and the time of its next sleep. –The new node follows the same procedure to choose schedule. The initial listen period should be long enough.

6. SNU Mobile Networks Lab. Periodic Listen and Sleep Timing relationship between receiver and different sender

7. SNU Mobile Networks Lab. Collision Avoidance Collision avoidance –The same procedure as 802.11. –To adopt RTS/CTS exchange and physical/virtual carrier sense. –Randomized carrier sense time.

8. SNU Mobile Networks Lab. Overhearing Avoidance To avoid overhearing by letting interfering nodes go to sleep after they hear a RTS or CTS packet. Who must go to sleep? –All immediate neighbors of both the sender and receiver should sleep. Sleep until NAV becomes zero.

9. SNU Mobile Networks Lab. Message Passing Only one RTS and CTS packet are used to send the fragmented long packet. –To avoid control overhead. To do overhearing avoidance.. –Each RTS/CTS/DATA/ACK packet has its duration field. –The duration field include expected transmission time of all fragment. –Sleep until NAV becomes zero.

10. SNU Mobile Networks Lab. T-MAC Fixed duty cycle like S-MAC, is not optimal. –The nodes must be deployed with an active time that can handle the highest expected load. –Whenever the load is lower than that, the active time is not optimally used and energy will be wasted on idle listening An active period ends when no activation event has occurred for a time TA

11. SNU Mobile Networks Lab. Cycle of S/T-MAC S-MAC T-MAC

12. SNU Mobile Networks Lab. Activation Event –The periodic frame timer. –The reception of any data on the radio. –The end-of-transmission of a node’s own data packet or acknowledgement. –RTS, CTS packet TA determines the minimal amount of idle listening per frame.

13. SNU Mobile Networks Lab. Choosing TA TA > C + R + T –C : the length of the contention interval –R : the length of RTS packet –T : the short time between the end of the RTS packet and the beginning of the CTS packet

14. SNU Mobile Networks Lab. RTS operation RTS retry –When a node sends an RTS, but does not receive a CTS back. (If a receiving node has not heard the RTS due to collision) –As the sending node receives no answer within the interval TA, it might go to sleep. –Therefore, a node should retry by re-sending the RTS if it receives no answer.

15. SNU Mobile Networks Lab. Early Sleeping problem T-MAC Problem

16. SNU Mobile Networks Lab. Solution Future request-to-sendTaking priority on full buffer

17. SNU Mobile Networks Lab. Simulation results

18. SNU Mobile Networks Lab. References W. Ye, J. Heidemann, and D. Estrin. An energyefficient MAC protocol for wireless sensor networks. In 21st Conf. of the IEEE Computer and Communications Soc. (INFOCOM), pages 1567 – 1576, June 2002. Tijs van Dam, and Koen Langendoen, An adaptive-efficient MAC protocal for wireless sensor networks. ACM, 2003